Cindi : Yes, that air in there thing happens enough that cis girls have named it a "queef."

You're quite right about what causes it, too - unlike a fart, it's just normal air getting trapped and then escaping later. Sex/dilation can do it, but so can just shifting around in just the right (wrong?!) way.
Now, one thing I did NOT expect was how actual farts (as in, I know they started out from my butt) will sometimes creep and bubble into my labia and kinda get channeled out and up through there instead. That is a *weird* feeling.
If I take a bath a little bit of water will get in, but not very deeply, and trickle back out when I stand up. It's not nearly as bad as the gush of lube after dilation, which is annoying as heck and also felt really strange at first. And Cindi's also right about washing the outside but not douching; I occasionally use a baby wipe for extra touch-ups in summer or after wearing tight pants, but other than that a quick wash of the labia with gentle soap in the shower is all I need.